Industrial adhesive tape applications very often use pressure sensitive polyacrylate adhesives. Polyacrylates possess diverse advantages over other elastomers. They are highly stable toward UV light, oxygen, and ozone. Synthetic and natural rubber adhesives usually contain double bonds, which render these adhesives unstable toward the aforementioned environmental influences. Another advantage of polyacrylates is their transparency and their capacity for use across a relatively broad temperature range.
Pressure sensitive polyacrylate adhesives are prepared generally in solution by a free radical polymerization. Generally speaking, the polyacrylates in solution are coated via a coating bar onto the carrier material in question, and are subsequently dried. In order to raise the cohesion, the polymer is generally crosslinked. Curing proceeds thermally, by UV crosslinking (ultraviolet radiation), or by EB curing (EB: electron beams). The operation described is relatively costly and inconvenient and is environmentally objectionable, since the solvent is generally not recycled, and a high level of organic solvent consumption entails a high environmental burden.
It is very difficult, moreover, to produce pressure sensitive adhesive tapes with high coatweight without bubbles.
An improvement in these disadvantages is provided by the hotmelt process. In this hotmelt process, the pressure sensitive adhesive (PSA) is applied in the melt to the carrier material.
This new technology, however, also entails restrictions. Before the coating operation, the solvent is removed from the PSA in a drying extruder. The drying procedure involves a relatively high temperature and shearing exposure, causing high damage particularly to high molecular mass polyacrylate PSAs. The acrylate PSA gels, or the low molecular mass fraction is increased greatly as a result of molecular weight reduction. Both effects are undesirable, being deleterious to use. Either the adhesive can no longer be coated, or else there are changes in the technical properties of the PSA, since, for example, on exposure of the adhesive to a shearing force, the low molecular mass fractions act as lubricants and so lead to premature failure of the adhesive.
A solution to avoiding these disadvantages is offered by polyacrylate adhesives with a low average molecular weight and narrow molecular weight distribution. Here, the polymerization procedure greatly reduces the proportion of low molecular mass and high molecular mass molecules in the polymer. The removal of the high molecular mass fractions lowers the flow viscosity, and the adhesive exhibits less of a tendency to gel. The lowering in the low molecular mass fraction reduces the number of oligomers that lower the shear strength of the PSA.
A variety of polymerization techniques are suitable for the production of low molecular mass PSAs. State of the art is the use of chain transfer agents (CTAs), such as of alcohols or thiols, for example. These CTAs reduce the molecular weight, but broaden the molecular weight distribution.
Another control polymerization technique employed is that of Atom Transfer Radical Polymerization, ATRP, where preferably monofunctional or difunctional secondary or tertiary halides are used as initiator, and certain metal complexes are used for the purpose of abstracting the halide or halides. As a side-effect, however, the metal catalysts employed generally influence the aging of the PSAs in a negative way (gelling, transesterification). Moreover, the majority of metal catalysts are toxic, discolor the adhesive, and are removable from the polymer only by costly and inconvenient precipitations.
Other controlled radical polymerization processes utilize a compound of the formula R′R″N—O—X, for example, as initiator, in which X represents a free radical species which is able to polymerize unsaturated monomers; very specific radical compounds, such as phosphorus-containing nitroxides or specific nitroxyls, for example, which are based on imidazolidine, morpholines, piperazinones, or piperazinediones. In general, however, the reactions exhibit low conversion rates, particularly for the polymerization of acrylates, leading to very low yields and molecular weights, and/or relatively high temperatures are needed in order to shift the equilibrium between the “dormant” and the active radical species to the side of the active species, and in order, thus, to increase the net reaction rate. The choice of solvents is therefore confined essentially to high-boiling solvents.
One suitable method for producing narrow-range polymers is that known as the RAFT procedure (Reversible Addition-Fragmentation Chain Transfer), described for example in specifications WO 98/01478 A1 and WO 99/31144 A1. The procedure described therein is not immediately suited to the production of PSAs, since the conversions achieved are very low and the average molecular weight of the polymers prepared is too low for PSAs, especially those based on acrylate. Consequently, the polymers produced in this way cannot be employed as acrylate PSAs.
Onward developments of this procedure, through the introduction of thioesters or trithiocarbonates, are a topic of research. For instance, EP 1 626 994 A1 describes improved RAFT CTAs, with which it has proved possible to produce polyacrylate PSAs for hotmelt coatings. The RAFT procedure has significant advantages over ATRP and over nitroxyl-controlled polymerization, since there is no need to accept a reduction in reaction rate, nor to use expensive and in some cases unstable catalysts, and the RAFT CTAs are more universal in their usefulness.
Generally speaking, nevertheless, acrylate PSAs obtained by means of sulfur-containing RAFT reagents of these kinds have disadvantages for numerous spheres of use. Polymers prepared with RAFT CTAs, especially (pressure sensitive) adhesives, do in fact have very defined polymer constructions and polydispersities, and can therefore be adjusted very effectively in terms of their technical adhesive properties. By the nature of the RAFT CTAs, however, because of the chromophoric sites of the functional groups (having conjugated double bonds containing especially sulfur atoms, optionally oxygen or nitrogen atoms), the polymers obtained are generally relatively intense yellow to brown in color, which is intolerable for a multitude of applications in the adhesive tapes sector. Moreover, production-related sulfur fragments in the polymers, by-products and CTA residues can have a very unpleasant odor and are typically themselves colored.
It is therefore an object of the invention to provide a corresponding polymerization method, by which polyacrylate adhesives having a narrow molar mass distribution can be prepared, but which has the disadvantages of the prior art cited only to a reduced degree, if at all. Significant disadvantages include the slow reaction rate of the ATRP and the nitroxyl-controlled polymerization, and also the odor and the color of the RAFT CTAs.
It has been found that, surprisingly, the combination of a RAFT process with subsequent addition of a diene compound after completion of the polymerization afforded polyacrylate pressure-sensitive adhesive systems that are virtually colorless and odorless, but were nevertheless prepared utilizing the advantages of the RAFT process, namely the high reaction rate and the narrow molar mass distribution.